The present invention relates to a novel NMR detector that allows the NMR spectra of self-assembled molecular monolayer and multilayer films, each prepared on a macroscopic and atomically flat metallic surface.
Traditional scattering techniques, such as x-ray reflectivity experiments, only determine the thickness of monolayer and multilayer films and have not been able to specify details about the arrangement and motions of molecules that compose the films or of guest molecules that are embedded in the films.
The modification of metallic surfaces using organic films, which are composed of highly ordered molecular assemblies, is a technological development that parallels the semiconductor revolution. Applications for ordered molecular films of nanoscale thickness will prevail in numerous nanotech areas, including flat panel displays, highly selective biological sensors, paints and industrial coatings, and magnetic storage. X-ray and electron diffraction methods are used extensively to characterize the rigid and periodic atomic architecture of semiconductor materials. Several additional electron scattering techniques are used to investigate semiconductor multilayers used as electronic gates and switches. However, diffraction and scattering instruments are of limited use for investigations of the fluctuating molecular architecture and dynamics of self-assembled molecular films. Therefore, a new approach is needed for investigating the conformations and motions of the molecules that compose monolayer and multilayer nanoscale films.
Rod and disk shaped molecules that spontaneously self-assemble to form periodic or aperiodic two-dimensional structures are the building blocks for the new emerging technology of nanoscale coatings. The intermolecular interactions that bind molecules together to form thin films are significantly weaker than those that bind the atoms in semiconductor materials. The most significant consequence of these weaker intermolecular forces is the substantial vibrational, librational, and rotational mobility of the molecules. Traditional diffraction and scattering techniques only elucidate the thickness of monolayer and multilayer films, and have not been able to specify details about the arrangement and motions of molecules that compose the films or of guest molecules that are embedded in the films.
Nuclear magnetic resonance (NMR) analysis is a powerful method by which to determine chemical structures and to examine reaction dynamics in a diversity of chemical and biochemical systems.
For example, U.S. Pat. No. 5,574,370, issued Nov. 12, 1996 to Woelk et al., discloses a toroid cavity detection (TCD) system for determining the spectral properties and distance from a fixed axis for a sample using Nuclear Magnetic Resonance. The detection system consists of a toroid with a central conductor oriented along the main axis of the toroidal cylinder and parallel to a static uniform magnetic field, B0. An RF signal is applied to the central conductor to produce a magnetic field B1 perpendicular to the central axis of the toroid and whose field strength varies as the inverse of the radius of the toroid. The toroid cavity detection system can be used to encapsulate a sample, or the detection system can be perforated to allow a sample to flow into the detection device or to place the samples in specified sample tubes. The central conductor can also be coated to determine the spectral property of the coating and the coating thickness. The sample is then subjected to the respective magnetic fields and the responses measured to determine the desired properties.
U.S. Pat. No. 6,046,592, issued Apr. 4, 2000 to Rathke et al., discloses a near-electrode imager for employing nuclear magnetic resonance imaging to provide in situ measurements of electrochemical properties of a sample as a function of distance from a working electrode. The near-electrode imager uses the radio frequency field gradient within a cylindrical toroid cavity resonator to provide high-resolution nuclear magnetic resonance spectral information on electrolyte materials.
U.S. Pat. No. 6,191,583, issued Feb. 20, 2001 to Gerald II, et al. discloses a toroid cavity detector that includes an outer cylindrical housing through which extends a wire along the central axis of the cylindrical housing from a closed bottom portion to the closed top end of the cylindrical housing. In order to analyze a sample placed in the housing, the housing is placed in an externally applied static main homogeneous magnetic field (B0). An RF current pulse is supplied through the wire such that an alternately energized and de-energized magnetic field (B1) is produced in the toroid cavity. The B1 field is oriented perpendicular to the B0 field. Following the RF current pulse, the response of the sample to the applied B0 field is detected and analyzed. In order to minimize the detrimental effect of probe ringing, the cylindrically shaped housing is elongated sufficiently in length so that the top and bottom portions are located in weaker, fringe areas of the static main magnetic B0 field. In addition, a material that tends to lessen the effect of probe ringing is positioned along the top and bottom ends of the toroid cavity. In another embodiment, a plug is positioned adjacent the inside of the top and bottom ends of the toroid cavity so that the sample contained in the toroid cavity is maintained in the strongest and most homogeneous region of the static magnetic B0 field.
U.S. Pat. No. 6,469,507, issued Oct. 22, 2002 to Gerald II, et al. discloses imaging apparatus used in a toroid cavity detector for nuclear magnetic resonance (NMR) analysis to hold samples relative to a principal detector element which is a flat metal conductor, the plane of which is parallel to the longitudinal axis of the toroid cavity. A sample is held adjacent to or in contact with the principal detector element so that the sample can be subjected to NMR analysis when a static main homogeneous magnetic field (B0) produced by a NMR magnetic device is applied to the toroid cavity and an RF excitation signal pulse is supplied to the principal detector element so that an alternately energized and de-energized magnetic field (B1) is produced in the sample and through the toroid cavity. The sample may be components of a coin cell battery which are mounted within the toroid cavity relative to the principal detector element by an non-conductive coin cell battery imager or a press assembly so that the components are hermetically sealed together and so that a direct current (DC) potential can be applied to the components. Alternatively, a sample is positioned within an O-ring maintained relative to the principal detector element by a pair of glass plates that are disposed on opposite sides of the principal detector element and are compressed toward each other so that NMR analysis can be used to analyze the sample with light transmitted through the sample or to analyze a sample separated from the principal detector element by semi-permeable membranes.
The subject matter of each of the U.S. Pat. Nos. 5,574,370, 6,046,592, 6,191,583, and 6,469,507 is incorporated herein by reference.
A principal object of the present invention is to provide a novel NMR detector that allows the NMR spectra of self-assembled molecular monolayer and multilayer films, each prepared on a macroscopic and atomically flat metallic surface.
In brief, the novel NMR detector of the present invention comprises a radio frequency (RF) resonance circuit. The RF resonance circuit includes a principal detector element and a sample chamber. The principal detector element defines an inductor of the electronic resonance circuit.
In one embodiment of the invention the sample chamber containing the inductor is a stainless steel sample chamber. The stainless steel sample chamber is a modified toroid cavity detector (TCD).
In accordance with features of the invention, the inductor is formed by an atomically flat metallic disk, such as, a mercury pool, with a predefined surface area, such as a surface area of 7.5 cm2. Liquid mercury is incorporated into a toroid cavity detector as the inductor of the resonance circuit, and as the base of the cavity.
Self-assembled molecular structures (monolayers and multilayers) are formed using long-chain alkane thiols, which are known to chemically react with silver, gold, platinum, and mercury surfaces. The NMR spectra of the alkane chain protons reveal an anomalous chemical shift of xe2x88x924.1 ppm for the methylene protons, which typically resonate at +1.25 ppm in isotropic solutions. In addition, rotating frame images confirm that the NMR signal originates only from the surface of the metallic substrate. Imaging results also indicate that the bound mercury alkane thiolate monolayer may impart molecular order in a liquid phase composed of excess neat alkane thiol and located directly above the monolayer film. These experiments represent the first NMR spectroscopy data on supported highly ordered nanothick films of macroscopic dimensions, and demonstrate that the novel NMR detector is functional.